Earth's nine life-support systems: Acid oceans

Boundary: Global average aragonite "saturation ratio" no lower than 2.75:1

Pre-industrial level: 3.44:1

Current level: 2.90:1

Diagnosis: Safe for now, but some oceans will cross the threshold by mid-century

This is a relatively new issue, rarely discussed until 10 years ago. More carbon dioxide in the atmosphere means more is absorbed by the oceans, creating carbonic acid. Since the industrial revolution, the pH of the ocean surface has fallen from 8.16 to 8.05, equivalent to a 30 per cent increase in the concentration of hydrogen ions.

Acidification per se isn't a problem, but it has serious knock-on effects on other aspects of ocean chemistry. The most important of these is that it lowers the amount of calcium carbonate dissolved in surface waters.

This doesn't matter much right now. But a critical point will be reached if waters become too low in aragonite, a form of calcium carbonate used by many organisms, including corals, to build their shells. Below a certain threshold, aragonite shells and coral dissolve in seawater.

So far, the average aragonite "saturation ratio" in the oceans has fallen from a pre-industrial level of 3.44:1 to 2.9:1. That means, on average, that there is still almost three times as much aragonite as is necessary to keep shells from dissolving. There are wide regional variations, however, and recent studies suggest parts of the Arctic and Southern Oceans could drop below the crucial aragonite saturation ratio of 1:1 by 2050.

Nobody knows quite what would happen then. Some species might be eaten away by the acidic water. It could be the coup de grâce for many corals already poisoned by pollution and bleached by warming waters. Emptier oceans would be able to absorb less CO2, accelerating global warming. To prevent any ocean waters from entering this parlous state, Rockström proposes keeping the average global aragonite saturation ratio above 2.75:1. That would mean keeping atmospheric CO2 levels below about 430 parts per million, which is lower than the 450 ppm that scientists say is the safe upper limit for global warming.

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